The thermal conductivity has been reported by Dudley to be 0.0135 cal. per second through 1 c.c. per 1°C. temperature difference, between 445 and 830°C. On the other hand, Goerens and Gillis found this value to be 0.0117 between 100'to 200°C., 0.1 between 0 and 1,000° and 0.008 at 1,000°C. Dougill, Hodsman and Cobb found conductivity to decrease rapidly with increasing temperature and computed the mean value between 0 and 1,000°C. to be 0.0155. The conductivity at 1,000°C. was 0.0085.
The linear thermal expansion expressed in per cent of the original length and the average of the results of three observers is 0.24 per cent at 200°C.: 0,51 at 1,200°; 1.88 at 1,400° and 1.88 at 1,600°.!
Magnesite bricks do not resist sudden temperature changes well and show a decided tendency to spall under such conditions. This type of refractories in contact with clay is attacked vigorously by the latter at furnace temperatures. Silica materials attack it likewise though less violently. Clay and magnesite and silica-magnesite refractories must therefore be kept from coming in contact by means of a neutral parting such as chrome brick or cement. Carbon likewise reacts with magnesia at high temperatures as does phosphorus.
Magnesite bricks show in the cold state a compression strength of approximately 2,000 to 3,000 Ib. per square inch. Their resistance to abrasion and impact is not marked owing to the comparatively weak bond. The electrical resistivity of magnesite refractories has been found by Stansfield, MacLeod and McMahon2 to be 6,200 ohms per cm.3 at 1,300°C.; 420 at 1,400°; 55 at 1,500°; 30 at 1,550° and 25 at 1,565°.
Dolomite Refractories.—The scarcity of magnesite during the war has brought about the marketing of highly burnt impure dolomites or crushed stone of this character treated with slag to make them more resistant to hydration. This material has been used quite successfully for open-hearth furnace bottoms but attempts to make from it bricks, corresponding to magnesite bricks, have not been fruitful. Dolomite refractories, while they may replace magnesite for furnace bottoms, cannot be said to equal the latter. Storage usually hydrates the lime to a more or less pronounced degree, thus lowering the mechanical strength of the product and in extreme cases bringing about its disintegration.
Spinel Refractories.—It is possible to produce a series of refractories consisting of magnesia and alumina combined to form the spinel, MgO.Al203 or containing an
excess of either constituent. Such a combination reduces materially the sintering temperature required for attaining constancy of volume of the calcine, especially in the presence of such impurities as occur naturally in magnesite and bauxite. An inspection of the fusion diagram of the system MgO.Al203 (Geophysical Laboratory), given in Fig. 11, shows the fusion point of the spinel to be 2,135°C. and that of the eutectic between this compound and MgO to be 2,060 .
1 From manuscript of HOWE and MCDOWELL, previous to publication in Journ. Am. Ceram. Soc:
2 Trans. Am. Electrochem. Soo., Vol. 22, p. 89 (1912
Minerals: 5 - Spinels
FIG. 11.—MgO-AhOs fusion diagram.